Liquid level indicating devices



l. B. JOHNSON I LIQUID LEVEL INDICATING DEVICES Oct. 13, 1959 Filed April 1, 1954 3 Sheets-Sheet l Inventor B WM A Horn ey5 Oct. 13, 1959 1. B; JOHNSON 2,908,166

I LIQUID LEVEL INDICATING DEVICES Filed April 1, 1954" s sheets-sheetz I 1 6 \JOHNJON Inventor A tt'orney Oct. 13, 1959 B. JOHNSON LIQUID LEVEL INDICATING DEVICES 3 Sheets-Sheet 3 'l-III inventor United States Pat Ofiicc oiiifalii LIQUID LEVEL INDICATING DEVICES Ivan Boris Johnson, Guildford, Surrey, England, assignor to Smiths America Corporation, Ruxton, Md.

Application April 1, 1954, Serial No. 420,409

11 Claims- (Cl. 73-304) 7 The present invention relates to devices for giving an automatic indication of the value of the ratio of two impedances and is particularly applicable to liquid contents gauges of the capacitance type, in which the capacitance of a plurality of capacitors (the tank' capacitors) distributed throughout a space to be occupied by a liquid varies substantially linearly with the volume of liquid in said space, so that the ratio of the said capacitance to a standard capacitance provides an indication of the quantity of liquid. 1

According to the present invention a device for giving an automatic indication of the ratio of first and second impedances of the same kind comprises a high gain am plifier having first and second input terminals, a first A.C. source having first and and second terminals, the first terminal of saidsource being connected to the first input terminal, the second terminal of said source being connected to the first terminal of the first impedance and the second terminal of the first impedance being connected to the second input terminal, and the second impedance being connected between the amplifier output and input to provide negative feedback,'whereby the net input to the amplifier is maintained substantially zero and the amplifier output varies substantially linearly with the ratio of the first and second impedances and provides the requisite indication.

By impedance of the same 'kind we mean impedances which are both capacitative or both inductive.

Means may be provided to stabilise the amplitude of the source, when the amplifier output may be utilised directly to provide the requisite indication. Alternatively,

- tance C, provided by the tank capacitors.

the voltages .respectively proportional to the first A.C.

source and the amplifier output may be applied to a ratiometer, when the ratiometer will provide the requisite indication.

When applied to a liquid contents gauge of the type referred to one impedance is provided by the tank capacin tors and the other is provided. by reference capacitor means. i

Preferably the reference capacitor means comprise one part whose dielectric is constituted by a representative sample of theliquid to .be gauged, whereby the indication provided is compensated for the effects of varying dielectric constant. Conveniently in such a device the capa'citance of thereference capacitor means may be'dependent upon the density of the liquid to be gauged,

whereby the device is also compensated for the effects of 5 varying liquid density.

, Alternatively, when the device is required for use with a range of liquids such that there is afunctional relationship between their density and dielectric constant, the

reference capacitor means comprise .a'capacitor whose dielectric is constituted by a sample of liquid in parallel with a fixed capacitor, said capacitorsjbeing such that .variationin capacitance of said capacitor means approximates to the requirements of the functional relationship for values of the dielectric constant in the region of a typical value thereof whereby the device is substantially compensated for the efiects of density and dielectric constant for liquids of said range. r

Embodiments of the invention, in which it is applied ts'liayi s stam a ge 9 the kmimffimd to wi l embodiments are derived.

Figure 2 shows a block diagram of a first preferred embodiment, particularly useful when one side of the tank capacitors is grounded. r

Figure 3 shows a circuit diagram of the first preferred embodiment.

Figure 4 shows a modification of the first preferred embodiment. v

Figure 5 shows a further modification of the first preferred embodiment. l

Figure 6 shows a block diagram of a second preferred embodiment.

Figure 7 shows a circuit vdiagram fcrred embodiment.

Referring toFigure 1, a high gain amplifier 1 has first and second input terminals 3 and 4 and first and second output terminals 5 and 6. An A.C. source 7 of amplitude vv and frequency a 7 Znhas one terminal connected to terminalS, while its other terminal is connected to one terminal of variable capaci- The other terminal of C is connected to terminal 4, standard capacitance E is connected between terminals 6 and 4, while terminals 5 and 6 are connected to an indicating meter 2. Terminals 5 and 3 are connected together. The sense of the amplifier output is such that the feed-back through E is negative. i

In the usual way the system attains a condition in which the input to amplifier 1 is substantially zero. Itis obvious that, in this condition,

enables the eflect of-st ray capacitances to be removed,

and also is practicallyconvenient in that'it enables one side of C and one output terminal of the amplifier'to be grounded. In this figure (and in succeeding ones) components equi'valent'to thosein Figure-1 are indicated by the same numeral. Terminals Sand 5 arenotfconnected together, and terminal 5 is grounded, as the side of C remote from terminal4. A further source '8, of amplitude 2v and in phase with "7, hasone terminal grounded and its other terminal connected through a further standard capacitance F to terminal 4.

The operation of this arrangement m ay be considered as follows: j

Suppose q is the volume of liquid, K is the dielectric constant of the liquid, jwx'is the current in thej'tank capacitors, joy is the current in the second standard capacitance (F); Then C=a+b(K-l)q where a 'and b are constants and we have, by obvious application of Kirchoifs laws:

of the second prei.e. the output voltage is proportional to the :liquid 1zol=urne. Y L. f

Referring now to the circuit diagram of EigurelBgthe deviceaobtains itsi DLC. .rpoweri supply: fromilines s9t(:positive) and 10 (earth). A triode valve V2b iszconnected ima conventional -Har-tleyzoscillaton circuit; the frequency-deterrnining components being capacitoruGMafid-CS andttapped inductor. L2, :giving :a frequency of about 50 kc./s. C7 and R7 indicate the conventional rgrid condenser andi leak, awhile ccathode .biasxisbprovided by resistors R8 and R9 and capacitor C8. .Thecathodmcun rent returns ito groundtviaipotentiometer RVZ, this being by'e'passed' :by J capacitor-x636. :A r first-i terminal of L2 .is connected to the grid of valve VZa. The cathode of valve V2a is returned to earth via \a neon stabiliser N1, which is by-passed by capacitor C1 and whose nongrounded terminal is connected to line 9 through resistor R1, The cathode of V2 a is connected through a resistorilu toahe slider tif'RVZ. Thus 'the amplitude of llatiousjacro'ss I Z2j war be such as jufst'to drive the g of va1veV2a 'pos'itivejwith respect to 'the cathode. voltage 'at the second terminal of I32 'vvill depend u n the setting of RVZ, so this will provide anadju'st their er 1116 amplitude or "oscillation.

(1L2 ,is'provided with two taps 11 and'12 between its 'centi'e' tap'and second terminal. The number of turns 11 and the. second terminal is equal to half ithafbetween 12 and' the second terminal. "Taps 11 and '12 constitute respectively the .non+grounded' terminals of the sources 7 and 8 of Figure'2.

Tap 11 is connected through a coupling capacitor C12 to the cathode of a triode'valve V3a, the cathode being returned .to 7 line 1 -10 through resistor. R13. Tap .12 is connected through ,parallel. capacitors C9 (having a pre set adjustment)=-and-C10in; series with capacitor C11.,to theZ- giid of V34 this being retumed to lineld through a: I e orP-Rll). "Tap 12 1's alsoconnected to ground;thr ough a pre-set potentiometer RV3 whose slider is. connected to The manner in which the circuit operates will be obviousfrom-comparison with Figure 2. -It-will-beseen that C9 (affecting F) is adjusted to ensure that the reading of meter 2 is correctly zeroed, while C14 (affecting E) is adjusted to ensure thatpthe scale of meter 2 bears the appropriate relationship to the quantity of liquid.

whilcl-itihasjzheen-..statQ11.1 at=; hc/; source fijhasl-m tude 2y, it will be appreciated that it may have any con: venient value, difierent values necessitating ydifierent values of'F. n e

The 1; purposecof FRYB; isto rpr vide. tarsmallicert n of the phase of .the voltage at point16, necessitated by V the relatively poor powerfactor of the tank capacitance.

Figure 4 showsone way inwhichthe circuit of Figure 3 may be modified .t o .,compensateforthe effects of varia: tion in dielectric constant of the liquid; C13 and'C14gare replaced by a .icapacitoritlwhose dielectric is,1cpnstitutedby a Irepresentativejsample of the jliquidj 15; while ;a trimming capacitor 21 is connected between point 18 arid point-16 It will be seenf'that 'theffeed'ba'ck provided through ?2,1 is posi't ive. The ca'pacitance oi 20 is of'the f rrn.g+ "1). W e es a d h a e; onst sngfbeing the'value of the capacitance'of '20 when the liquidjs remcved p us. st y apac ta ce and h.. beingia con t dependent upon th .,gcomet y.cf20- f-ll sasliustcd 5.0 that the positive feedback thr ugh it c ianc i he Lacs? tivet feedac t mu' cap ci ance g The. etfv effectively remake 1B (or figure 2) ncqualfltof bolt-Keel).

arid the yalue'iV (and reading ,of..'n,1eter 2) willvary linearlywithql irrespectiveofithe valueof i v A furthertm' dificationjis-shown inEigureQS. somewhat similar to. the arrangement of EigurAQexcept that .capacitoriZOis replaced byafloating referbl ceflcom denser. such. asiisl disclosed. in British.-. sp cification. No. 6731988. I he expression for. E then becomes 7 n(Kp- 1) where m and n are constants and p is the) density ofthe iq d- By s ita le adj st en V can" be. m de. tuva y 'linea'rlyrwith ;gq, i.e. with the mass, of liquid f 7 Referring now to; Figure I16 (showing 1 hlQQk .3813

' th second, pr ferred embodime c rtain of the w 3113.15 of, the secondary ofTl areconnectedthrough similar rectifiers MR2,MR3, to the primary winding of. afurther transformer l T2. Equal resistors R13, R19 are connected across the primarypf T2, .and; a-.milliamm eter- 2 is connected between theinjunction andzthe centreetap of the'secondary of T 1."The secondary of T2, is centretapped' to earth, and tuned by capacitor C17. .One extreme terminal; 17, of the SecondaryloflTZ-ds .con-

nected through fixed capacitor C13, and preset adjustable I capacitor C14 to point '16, this terminal being that which provides negative feed-backf'for the amplifier constituted by V3a, V312 etc. The other extreme terminal, 18, is

' not used in this embodiment. The voltage between 17 and ground is thus V, and the reading of meter 2 will be proportional to it. It will b e 1seen that C13 and C14 provide the capacitance E of Figure 2.

- V v willbe proportional component herelshown. cor-r pond P ciselyI ith. po'nentsf indicated by corresponding numbers or.;letters "in'TFig'ure, 1. However, indicatorz2fiof .Eigure. 1, ,is,re placed bya ratiometer indicated-at. 2a.,in Figure..6 R? which are applied the amplifier output V and .the. volt.- age source 7, A .furtherz-s ourcenzfi, of:the..sarne.amplitudeas source37f and in anti-phasehtherewith is .connected between, 5 ground and terminal 4, through 7 as eapacitance G. As mentioned earlier C is of the form adjusted so thatItherfeedthroughitto terminal-Accountaactsjflthe .feed through-the .fempty capacitance .of the tahlgfcapacitorsand the, associated strays. Thustwelzshall b(K-'-1)q- :E' I fit frequentlfhappens.'that,-when anumber o'fygfener'a'lly similar-liquids are, dealt with -(fo'r. .cxample aircraft engine. Ifuels'.) jtheir .d ielect ric constant t and density .arcfcloses'jly related an empirical ;formula of. theforni '.;K;.=.rp.+sawhere.r. and.s,arelconstants. Thus 'if; in-such a case capacitauce E cari be sofchosen toib pmr i rm' a; V a

- *'=K.;s" I e top q, iteiitq the massof liquid i i i hs limits "st a er sf hef fl approximation to this law of variation-maybe-obtained sthus' by utilising, to provide B, an air-dielectric capacitor in parallel with a capacitor whose dielectric is constituted by a representative sample of the liquid. .Such an arrangement has a capacitance t+uK,' where t and u are constants, and if t and u are so chosenthat A It will be seen that this amounts to the replacement of the expression I K-s by an expression which is equal to it when K=K varies linearly with K .and has a rate of variation with K equal to that of when K=K Referring now to the circuit of Figure 7 components there designated by numbers and letters as components in earlier figures are equivalent to them.

V2b is connected in a circuit similar to that of Figure 2, except that an intermediate tapping, 24, of inductor L1, is grounded. The terminal 26 of L1 remote from V2b grid consitutes the non-grounded terminal of source 7, and a further tapping 25, such that the voltage thereat is equal in magnitude and opposite in phase to that at 26, constitutes the non-grounded terminal of source 23. 26 is connected to one side of the tank capacitors 13 while the other side of the tank capacitors 13 is connected, via a coupling condenser C18 and grid leak R20 to the grid of triode V4a. The junction of the tank capacitors and C18 is thus point 4. V4a is connected in a conventional amplifier circuit and its output is fed to triode V4b which is similarly connected. The anode load of V4!) is the primary of a transformer T3, having a centre-tapped secondary, the centre tap being grounded. The secondary is tuned by a capacitor C19. One secondary terminal point 17 is connected to point 4 via a fixed capacitor C20 and a capacitor C21, filled with a representative sample of the liquid in tank 14. These are designedto fulfill the conditions set out above, and provide capacitance E. Terminal 17 is also connected to one winding of an A.C. ratiometer 2a whose other winding is connected to tapping 25. A fixed capacitor C22 and preset variable capacitor C23 are connected in parallel between tapping 25 and point 4. These provide capacitance G.

The manner in which the device operates will be immediately obvious from the earlier description with reference to Figure 6. C23 is of course adjusted to cancel out the effect of the empty tank capacitors and the associated strays. It will be clear to those skilled in the art that source 23 need not necessarily be equal in magnitude to source 7. If it is not, capacitance G will not be equal to the empty tank capacitance.

It will be appreciated that the reference capacitors (providing capacitance E) described with reference to the either preferred embodiment may be used in the other with appropriate modifications which will be obvious in the light of the description. Also the amplitude-stabiliser circuit of the first circuit could be omitted and a Iatiometer indicator used, or an amplitude-stabiliser cir- -cuit could be incorporated in the second embodiment and a straightforward meter used in place of aratiometer as the indicator. 7 v It will also be appreciated that, in all of the embodiments, the reference and standard capacitors may be interchanged, with consequential alterations to there mainder of the circuit which will be obvious in the light of the earlier description. Such arrangements will not be quite so convenient as those described, however, if a straightforward meter is used as indicator, as then the reading will be inversely proportional to liquid contents.

It will be seen that, by the use of devices in accordance with the invention, automatic impedance indicators are provided which do not require any mechanical mov ing components, in distinction from various prior proposals entailing the use of a servo motor and a standard impedance such as a potentiometer, actuated thereby.

I claim:

1. A device for giving an automatic indication of the ratio of first and second impedances of the same kind comprising a high gain amplifier having first and second input terminals, a first A.C. source having first and second terminals, the first terminal of said source being connected to the first input terminal, the second terminal of said source being connected to the first terminal of the first impedance and the second terminal of the first impedance being connected to. the second input terminal, and the second impedance being connected between the amplifier output and input to provide negative feed-back, whereby the net input to the amplifier is maintained substantially zero and the amplifier output varies substantially linearly with the ratio of the first and second impedances and indicator means responsive to the amplifier output, the said indicator means providing the requisite indication.

2. A device as claimed in claim 1, together with means to stabilise the amplitude of the AC source, and the indicator means provides an indication proportional to the amplifier output.

3. A device as claimed in claim 1, wherein the indicator means is a ratiometer, means to apply to the terminals of said ratiometer voltages proportional re spectively to the amplitude of the first source and the amplitude of the amplifier output, whereby it provides the requisite indication.

4-. A device as claimed in claim 1 wherein one impedance is provided by the tank capacitors in a liquid contents gauge of the type referred to and the other is provided by reference capacitor means.

5. A device as claimed in claim 4 wherein the reference capacitor means comprise one part whose dielectric is constituted by a representative sample of the liquid to be gauged, whereby the indication provided is com pensated for the effects of varying dielectric constant.

6. A device as claimed in claim 5 wherein the capacitance of the reference capacitor means is dependent upon the density of the liquid to be gauged whereby the device is also compensated for the efiects of varying liquid density.

7. A device as claimed in claim 4 for use with a range of liquids such that there is a functional relationship between their density and dielectric constant wherein the reference capacitor means comprise a capacitor whose dielectric is constituted by a representative sample of liquid in parallel with a fixed capacitor, said capacitors being such that variations in capacitance of said reference capacitor means approximates to the requirements of the functional relationship for values of the dielectric constant in the region of a typical value thereof, whereby the device is substantially compensated for the effects of density and dielectric constant for liquids of said range.

8. A device as claimed in claim 4 wherein the first impedance is provided by the tank capacitors and the second impedance by the reference capacitor means.

- dereloped as a voltage between said terminals,- hel firs t mammal a t; he re erenc anag tor-meam iswwnn c to hns qqnd.-.am fi xinp 't t zm nalrth se b wte r A 3H ininalioi-s idretictiw sapa i o m sti 'ct nectq to 0n e fw e firstmsb omiaiup ifi n.qu putjtemii ,-the.seaondnamril fie out? of a pl ude whieh is at pnst ntimylt plsqtthat f tegminaLjsQe nne ted'to thesecondterminal of the the first-sourcqeonnected in seiieswithm'fm'ther standfi t-sonrse ndsa secon sou ce o vtheisam f eq e cy an r n her be ween! he-first-iand: sesqs li ss tzse and a eha in sfi @mp1it d ..gr% rt anthefir t min s:\W Y:T hQ1 fi@i,=If fimililt:zial'liififiifii ifiwfi un hvafiwdmult p hewoi, isuconnes s ries may be nfintra wi a W t fu ther ljstand rdwcapaci b ween t v e o a a Le: alrpfthqfirstr qurq tand the se ond amp fie References Cited in h file' this P iif" Butt "1 2ml sai nturther standa d-repeat .sewingflto V UNITED SIATES PATENTS dete mine he 1. m Q th indieatiqn pro ided a 2 377 275 i v Ma I I 1 -1-.' y Ad ylc a r ala r fledw m d um 9W1JI@1.I1 the 2,560,709 lworodward I July 17, 1951 p e 'i' ashal o alth dp tpu t r mn l, th utputs o 2,581,085

age between which third terminal and the segond ter- 1952 Edelman 2,621,517 Sontheimer minal being in antiphase with the voltage between the 1 first, and second terminals, comprising also a trimming b 9 g capacitor connected between said third terminal'andthe 541 M 1i "3- :1 4 -"195 5 second amplifier input terminal, whereby unwanted nega- 2724273 S 1 g l 1955 tive ,feed back :through the reference capacitor means 2728035 fig i g D :c 20 1955 may be neutrahsed' 2,737,808 Moss' -fts-mara mus s 11. A'deviee as claimed in claim 8 whereinthe am- 25 V 1 p'l'ifierhas' two output terminals the amplifier output campam Marlvfiq lQfifi being developed as a voltage between said ,terminals, the I 

